For office workers who suffer back and neck aches, the culprit may be right underneath their noses... well, underneath their posteriors anyway.

Chairs that force the user to lean too far forward or back and those set at the wrong height are among the most commons causes of painful back and neck strain. They also contribute to leg, arm and shoulder problems, says Cheryl Resnik, D.P.T., director of physical therapy at USC University Hospital.

"Having a properly fitting chair is important if you sit for a long period of time. People who sit at their desk all day and don't take breaks are at the greatest risk for those kinds of problems," she says.

Preventing muscle strain and the pain that goes with it is fairly simple however.

Chairs seats should be low enough to the ground or accompanied by a foot rest so that the legs don't dangle and restrict circulation. But chairs should also be high enough so they permit the hands to rest comfortably on a keyboard with the elbows bent at a 90 degree angle.

Because most chair seats are about 18 inches from front to back, shorter people often compensate by sitting and leaning farther forward, which increases the sway in the lower back. For others who sit with their bodies close to their desks but tilted backward, the spine loses its natural curve in the lower back. Both positions tend to cause strain and aches, Resnik says.

Maintaining a poor posture for long periods of time may also contribute to a reduction in mobility and strength. As a result, when someone needs to move quickly in a surprise situation, such as a trip or fall, they are more likely to be injured.

Resnik advises anyone who works at a desk job to have an ergonomics expert examine their work area and also make sure they take frequent breaks to stretch and get away from their keyboards.

She also adds that chairs designed to promote good posture-such as kneeling chairs that have no back and balance a person's weight between their knees and posterior-do not necessarily work.

"They're like everything else. They have to be evaluated for the individual," she says.